Los Angeles: The family of a woman allegedly thrown to the ground and punched repeatedly by a California Highway Patrol officer is asking that the two officers involved be punished, the woman's lawyer said.

"She's not just some animal," said Caree Harper, who declined to name the woman. "She has an aunt, a sister, a brother, a father and a great-grandchild."

California Highway Patrol officials launched an investigation after video of the altercation was broadcast on television news programs.

A still from a passing motorist's video shows the confrontation between a California Highway Patrol officer and a woman on a Los Angeles freeway. Photo: AP

The video shows the woman walking on the shoulder of a freeway west of downtown Los Angeles when a Highway Patrol officer on foot catches up with her, spins her around and throws her to the ground. As he struggles to subdue her, the officer can be seen on the video punching her at least eight times in the head while she is pinned on her back.

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A plainclothes officer appears and helps to handcuff her.

Passing driver David Diaz recorded Tuesday's incident and gave it to media outlets.

He said he arrived as the woman was walking off the freeway. He said she turned around only after the officer shouted something to her.

"He agitated the situation more than helped it," said Mr Diaz, who started filming soon after.

The extent of the woman's injuries was unclear, but Highway Patrol officials said she was not injured and she was having a mental health evaluation. The woman's name would not be released, officials said.

The woman's doctors were not allowing the family to see her, Ms Harper said.

"There is no justification for the way that he savagely beat her," Ms Harper said. "He's the one that should be in a mental health facility."

Highway Patrol officials said the video captured only a small part of the incident. The officer was trying to stop the woman from walking into traffic and endangering herself and others, they said. They would not identify the officer, but said they would have a full investigation.

The women's relatives had not contemplated a lawsuit because they hadn't been able to determine her condition, Ms Harper said. She declined to say why the woman was walking on the freeway, but said nothing could justify the officer's tactics.

"We are not going to let this woman be on trial," said Harper. "We're going to turn the attention on the wannabe MMA [mixed martial arts] cop who repeatedly punched her as she was lying on her back."